Parasites of Skipjack Tuna, Katsuwonus Pelamis: Fishery Implications

The numbers of 26 types of parasites were counted in 878 fish, of which all but 3 we.e from 14 areas in the Pacific. Data from the 22 most reliable parasites gave no evidence of discrete stocks of skipjack tuna in the Pacific, either when analyzed singly or when using combinations of parasites in multivariate analyses. New Zealand fish carried many tropical parasites, particularly didymozoids, in numbers similar to fish caught in the tropics, indicating that the bulk of these fish had recently migrated from the tropics. The number of Ten!(J!'U{(J"';(J coryphaena.e, a larval tapeworm, was positively correlated to fish size in the tropics. In New Zealand, however, fish over 55 em carried about the same number of T. corypha.enae as fish 45 to 55 em, SUI!;l.;csting they had left the tropics when they were 45 to 55 cm and had not returned. Analysis of the numbers of parasites from particular schools suggested that school members stayed together for several weeks but not for life. The use of parasites to delineate stocks for management purposes is a well-established technique. For a comprehensive review of the many examples see MacKenzie (1983). The skipjack tuna, Katsuwonus pelamis, is one of the most valuable fishery resources of the central and western Pacific. At least 50 species of parasites have been reported from it. The distribution of only one, the hemiuroid digenean Hirudinella ventricosa, has previously been investigated. In the Atlantic, Watertor (1973) found it in 7% of skipjack tuna off West Africa, 40% off Brazil, and <1% off Florida. In the Pacific, Nakamura and Yuen (1961) found it in 21 % of skipjack tuna off the Marquesas and 34% of fish from Hawaii. Sindermann (1961) pointed out that analyzing the distributions of combinations of parasites may provide more information than the examination of individual species. That, in general, has been our approach here. In addition, school-school variation in parasite numbers was studied to determine how long schools stayed together, and secondarily to evaluate the degree of permanence of the parasites. MATERIALS AND METHODS Of the 878 fish dissected, 386 were collected by the HatsutoriMaru on charter to the South Pacific ComlDepartment of Parasitology, University of Queensland, St. Lucia, Brisbane, Australia 4067. 'Faculty of Science, University of Queensland, St. Lucia, Brisbane, Australia 4067. 'Fisheries Research Division, New Zealand; present address: Southpac Fisheries Consultants, P.O. Box 7230, Auckland I, New Zealand. Manuscript accepted November 1984. FISHERY BULLETIN: VOL. 83, NO.3, 1985. mission (SPC), 246 by the New Zealand Ministry of Agriculture and Fisheries (NZ), and the remainder by other governments and fishing companies (see Acknowledgments). Fish were obtained from 15 areas (Fig. 1, Table 1). Gills and viscera were frozen and flown to Brisbane for dissection. The SPC and NZ fisheries officers sampled 5 fish/school from a maximum of 3 schools/d. Commercial companies were unable to sample from individual schools and usually supplied the head and the anterior ventral body, removed from frozen fish by a single slanting cut using a band saw. Fork length, if not supplied, was calculated TABLE 1.-Sources of fish dissected. Avg. No. length Area Date fish (em) A Palau, Helen R. Aug. 1980 35 41 B Pan ape July 1980 45 59 C Papua New Guinea June 1981 30 50 D Papua New Guinea Nov. 1981 60 41 E Solomon Is. June 1980 30 46 F Coral Sea Jan. 1982 19 57 G Fiji Feb., Mar., Apr., May 1980 100 50 H Norfolk Is. Mar. 1980 21 57 I New South Wales Jan. 1981 103 47 J New Zealand, west Mar. 1980; Jan., Feb. 1982 69 52 K New Zealand, east Jan. 1980; Jan., Feb. 1982 163 49 L Marquesas Dec. 1979; Jan. 1980 150 47 M California Aug. 1981 30 47 N Ecuador Jan. 1982 20 48 0 Atlantic Mar. 1981 3 50 (Puerto Rico)